The present invention relates generally to semiconductor devices. More specifically, the invention relates to ion implantation used in making semiconductor devices.
In the formation of semiconductor devices in integrated circuit chips, an isolation edge region may be formed to separate an N-doped region from a P-doped region.
To facilitate discussion, FIG. 1 is a schematic view of an isolation junction between an N-doped region 104, such as an N-well, and a P-doped region 108, such as a P-well, in a substrate 112. The N-doped region 104 may be formed by an ion implantation of an N-type dopant. The P-doped region 108 may be formed by an ion implantation of a P-type dopant. A spacer 116 may be used to separate the N-doped region 104 from the P-doped region 108. Both the N-doped region 104 and the P-doped region 108 extend under the spacer 116 due to lateral straggle of the implanted dopant and due to the lateral diffusion during temperature cycling. Lateral straggle is caused by implantation of the dopant in the substrate. Both lateral implant straggle and lateral diffusion may cause the N-doped region 104 to have a bowed shape 120 that extends under the spacer 116. In the prior art, lateral straggle may be about 0.7 times the depth of the junction while diffusion will cause dopant to move laterally as much as it does vertically. So for a junction with a 0.5 micron depth, the lateral straggle may be 0.35 microns. Lateral diffusion causes even more lateral travel. Providing space to accommodate the bowed shape 120 of the N-doped region 104 and the lateral 2d dopant profile of the P-doped region 108, caused by both lateral straggle and lateral diffusion, may waste chip real estate.
If the space required by the NP isolation junction could be reduced, the resulting chip size may also be reduced. It is desirable to provide a more compact NP junction to reduce the size of a semiconductor device.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a variety of techniques is provided for forming an isolation junction between a region of a first conductivity type and a region of a second conductivity type in a substrate. Generally, a first series of dopings of a first conductivity type is provided. The first mask is then removed. A second mask is formed over a part of the surface of the substrate, wherein part of the surface covered by the first mask is not covered by the second mask and wherein part of the surface covered by the second mask was not covered by the first mask. A second series of dopings of a second conductivity type is provided, wherein the first series of dopings and the second series of dopings form a substantially vertical junction.
The present invention also provides a variety of techniques for determining a recipe for creating a vertical junction. Generally a first mask is specified. A first series of dopings is also specified. The removal of the first mask is then specified. A second mask is then specified. A second series of dopings of a second conductivity type is then specified. The resulting junction position is then determined. The first series of dopings and second series of dopings are then respecified to reduce a difference between the resulting junction position and a line through the junction.
The present invention also provides a region in a semiconductor substrate. The region has a P-doped region and an N-doped region in the substrate, where the N-doped region is adjacent to the P-doped region. A substantially vertical junction separates the P-doped region from the N-doped region.